1. Field of the Invention
This invention relates to a copier, and more particularly to the control of a sub-scanning unit.
2. Description of the Prior Art
Recently, copiers are required having a higher copying rate and higher picture-positioning accuracy for their various editing functions.
In the conventional copiers, the drive sequence of the sub-scanning unit, which mechanically scans an original sheet, is such that the sub-scanning unit accelerates from the home position (the position where the sub-scanning unit is normally waiting) until it reaches a specified speed, makes sub-scanning at the specified speed to the original image reading end position, and returns to the home position. The sub-scanning unit always operates in this sequence regardless of the position of the original sheet on the original table and regardless of the size of the original sheet. Even when a same image of one original sheet is copied repeatedly, the above drive sequence is repeated for each copying operation. That is, the sub-scanning unit is returned to the home position after each copying operation for the next sequence. Therefore, the copying rate is inherently limited because of the continuous repetitive copying operation. For example, a continuous repetitive copying rate of 30 sheets/min for the A4 size sheet cannot be attained.
Also, to achieve enlargement, reduction, or transfer copy function, the acceleration time of the sub-scanning unit for each of numerous copying modes must be previously measured. (Refer to, for example, Laid Open Japanese Patent Application No. 59-63876 and No. 61-45670.)
These problems of conventional copiers are due to the difficulty in accurately controlling the sub-scanning unit. The conventional sub-scanning units are controlled by the PLL control or by the open loop control of a stepping motor. Using these control methods, it is difficult to control the sub-scanning unit accurately and flexibly.